Principle of Single Junction Transistor

Single junction transistor (UJT), also known as base diode, is a semiconductor device with only one PN junction and two resistive contact electrodes. Its substrate is a strip-shaped high resistance N-type silicon wafer, and two base electrodes b1 and b2 are respectively led out by ohmic contacts at both ends. Use alloy method to create a P region as the emitter e on the slightly b2 side in the middle of the silicon wafer.

1、 Characteristics of Single Junction Transistors

The resistance between the two base electrodes b1 and b2 is called the base resistance: rbb=rb1+rb2

In the formula: rb1- the resistance between the first base and the emitter junction, whose value varies with the emitter current ie, rb2 is the resistance between the second base and the emitter junction, whose value is independent of ie; The emitter junction is a PN junction, equivalent to a diode.

If a positive voltage Vbb is applied between the two sided three pole bases b2 and b1, the voltage at point A is:

VA=[rb1/(rb1+rb2)] vbb=(rb1/rbb) vbb=η Vbb

In the formula, η - is called the voltage divider ratio, which is generally between 0.3 and 0.85. If the emitter voltage VE gradually increases from zero, the volt ampere characteristics of a single junction transistor can be measured

(1) When Ve<η Vbb, the emitter junction is reverse biased, the transistor is cut off, and the emitter has only a small leakage current Iceo.

(2) When Ve ≥ η Vbb+VD is the forward voltage drop of the diode (approximately 0.7 volts), the PN junction conducts in the forward direction, Ie increases significantly, rb1 resistance rapidly decreases, and Ve decreases accordingly. This characteristic of voltage decreasing with increasing current is called negative resistance characteristic. The critical point P at which a diode enters the negative resistance region from the cutoff region is called the peak point, and the corresponding emitter voltage and current are called the peak point voltage Vp, peak point current Ip, and peak point current Ip, respectively. Ip is the forward leakage current, which is the minimum current required to make a single junction transistor conductive. Obviously, Vp=η Vbb

(3) As the emitter current ie continues to rise, Ve decreases continuously. After reaching point V, Ve no longer decreases. This point V is called the valley point, and the corresponding emitter voltage and current are called the valley point voltage, Vv, and valley point current Iv.

(4) After passing the V point, the carriers in the semiconductor between the emitter and the first base reach saturation, so when uc continues to increase, ie slowly rises. Obviously, Vv is the minimum emitter voltage that maintains the conduction of a single junction transistor. If Ve<Vv, the transistor will turn off again.

2、 Main parameters of single junction transistor

(1) When the base to emitter resistance Rbb is open, the resistance between the bases b1 and b2 is generally 2-10 kiloohms, and its value increases with temperature. 

(2) The partial pressure ratio η is a constant determined by the internal structure of the diode, generally ranging from 0.3 to 0.85.

(3) The reverse voltage Vcb1 b2 between eb1 is open circuited, and at the rated reverse voltage Vcb2, the reverse withstand voltage between the base b1 and the emitter e.

(4) Reverse current Ieo b1 is open circuit, and at the rated reverse voltage Vcb2, the reverse current between eb2.

(5) The saturation voltage drop Veo of the emitter is the voltage drop between eb1 at the maximum rated current of the emitter.

(6) The peak current Ip is the emitter current at which the emitter voltage of a single junction transistor is at the peak voltage when it first conducts.